Fairfax County Police have opened a criminal investigation into recent vandalism at two rival high schools.

In a joint letter issued to community members at both schools Tuesday, Herndon High School principal William L. Bates and South Lakes High School principal Kim Brophy Retzer addressed obscene graffiti that was discovered at both schools last week, on school buildings and athletic fields.

“Criminal vandalism has occurred in recent days at both South Lakes High School and Herndon High School. These were not just innocent pranks. Graffiti was spray-painted on buildings and new turf fields causing extensive damage and requiring an expensive clean up,” the letter states.

At Herndon High last week, blue spray-painted obscenities and the number “666” was discovered on a school building. On Saturday at South Lakes in Reston, obscenities spray painted in white were discovered on the high school’s new $1.4 million turf field. A depiction of Herndon’s “hornet” mascot was also spray painted on a concrete section of the South Lakes stadium. Both schools are in the process of removing all the grafitti.

“We are working closely with law enforcement to hold the perpetrators accountable,” the principals’ letter said. “As discouraging and disappointing as these acts are, they in no way will dampen the enthusiasm and sense of anticipation our staffs and students feel as we begin a new school year. The individuals committing these crimes do not represent our schools or the rich competitive tradition we have between our students, staffs, and communities. They are instead using our schools’ names in the most cowardly manner possible.”

Police on Tuesday confirmed that a criminal investigation has been opened in both incidents and that suspects are being sought, but at this point it is unclear if the vandalism is the work of one group or individual, or if the earlier event at Herndon High School spurred the second at South Lakes as a retaliation.

Anyone with information is asked to call Fairfax County Police at 703-691-2131